[AA] Make ModRefInfo a bitmask enum (NFC)

Mark ModRefInfo as a bitmask enum, which allows using normal
& and | operators on it. This supersedes various functions like
unionModRef() and intersectModRef(). I think this makes the code
cleaner than going through helper functions...

Differential Revision: https://reviews.llvm.org/D130870

GitOrigin-RevId: b128e057c191a441e3778ffc872ffca943b2e5b1
7 files changed